git-rev-list.txt: fix Asciidoc syntax

git-rev-list.txt: fix Asciidoc syntax

Using '{caret}' inside double quotes and immediately following with a
single quoted word does not create the desired output: '<commit1>'
appears verbatim instead of being emphasized.

Use a litteral caret ('^') instead.

Also, remove the leading tabs in shell examples to bring them more in
line with the rest of the documentation.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>